Normally-off integrated JFET power switches in wide bandgap semiconductors and methods of making

ABSTRACT

Wide bandgap semiconductor devices including normally-off VJFET integrated power switches are described. The power switches can be implemented monolithically or hybridly, and may be integrated with a control circuit built in a single-or multi-chip wide bandgap power semiconductor module. The devices can be used in high-power, temperature-tolerant and radiation-resistant electronics components. Methods of making the devices are also described.

This application is a continuation of U.S. patent application Ser. No.11/000,222, filed on Dec. 1, 2004, now U.S. Pat No. 7,202,528, which isrelated to U.S. Patent Application No. 60/585,881, filed Jul. 8, 2004,and U.S. patent application Ser. No. 10/999,954, filed on Dec. 1, 2004.Each of the aforementioned applications is incorporated by referenceherein in its entirety.

BACKGROUND

1. Technical Field

The present invention relates generally to field effect transistors(FETs), and in particular, to such transistors formed in wide bandgapsemiconductor materials. Further, this invention relates to monolithicand hybrid integrated circuits comprising low-voltage control circuitryand to power switches built using the above transistors.

2. Background of the Technology

Wide bandgap semiconductor materials (with E_(G)>2 eV) such as siliconcarbide (SiC) or Group III nitride compound semiconductors (e.g.,gallium nitride or GaN) are very attractive for use in high-power,high-temperature, and/or radiation resistant electronics. Monolithic orhybrid integration of a power

transistor and control circuitry in a single or multi-chip wide bandgappower semiconductor module is highly desirable for such applications inorder to improve the efficiency and reliability of the system.

SiC smart power technology has been a topic of discussion in recentyears, but has experienced limited scientific investigation. Proposedsolutions have been met with skepticism relating to the operation ofboth the power switch and control circuitry.

Because of the fundamental differences in material properties andprocessing technologies, traditional Si or GaAs integrated circuit (IC)technologies such as Complementary Metal-Oxide-Semiconductor (CMOS) orDirect Coupled FET Logic (DCFL) cannot in most cases be easilytransferred to wide bandgap semiconductors. Several attempts atfabricating SiC NMOS and CMOS digital and analog ICs have been reportedin the last decade (e.g., [1], [2]). A monolithic CMOS integrated devicein SiC and method of fabricating the same is disclosed in U.S. Pat. No.6,344,663, [3]. Moreover, recent development in SiC Lateral DMOSField-Effect Transistors (LDMOSFETs) (e.g., [4]-[5]) theoretically allowfor the monolithic integration of MOSFET-based control circuitry andpower switches for use in Smart Power electronics. Various issues,however, limit the use of MOSFET-based SiC integrated circuits in theapplications where high-temperature and/or radiation tolerance isrequired. The first such issue is on-state insulator reliability as aresult of a much smaller conduction band offset of SiC to SiO₂ ascompared to that of silicon. This issue becomes even more significant athigh temperatures and in extreme radiation environments. Other issuesinclude: low inversion channel mobility due to high interface statedensity at the SiC/SiO₂ interface and high fixed charge density in theinsulator; and significant threshold voltage shift with temperature dueto ionization of interface states.

Another transistor candidate for use in SiC Smart Power electronics, aSiC bipolar junction transistor (BJT), also suffers frominterface-related issues such as high recombination velocity on thesurface between the emitter and the base resulting in low current gainand high control losses.

Another transistor candidate for use in SiC Smart Power electronics is aMetal Semiconductor Field-Effect Transistor (MESFET). Despite the factthe SiC MESFET monolithic microwave integrated circuits (MMICs) receivedsignificant development in the last decade (e.g., [6]), there have beenfew published attempts to build SiC MESFET logic and analog circuits(e.g., [7]).

An alternative to the MOSFET and MESFET approaches is the use of lateralJFET-based integrated circuits implemented in either complementary(n-type and p-type channels as disclosed in U.S. Pat. No. 6,503,782 [8])or enhanced-depletion (n-type channels) forms. SiC JFETs have proven tobe radiation tolerant while demonstrating very insignificant thresholdvoltage shift with temperature. Encouraging results in the developmentof high-temperature normally-on power vertical junction field-effecttransistors (VJFETs) have been published in recent years (e.g., [9]).However, despite their excellent current-conduction and voltage-blockingcapabilities, a major deficiency of these transistors is that they are“normally-on” devices. On the system level, this often requires anadditional (negative) supply voltage and short circuit protection.

Several attempts to build normally-off SiC high-voltage VJFET switcheshave been reported recently. Typically, these devices comprise bothlateral and vertical channel regions (e.g., [10]-[12]). These devices,however, exhibit a drastic contradiction between the device blockingcapabilities and the specific on-resistance. For example, a VJFET with a75 μm, 7×10¹⁴ cm⁻³ n-type drift region was able to block above 5.5 kV atzero gate-to-source voltage [13]. At the same time, this devicedemonstrated a specific on-resistance (R_(sp-on)) of more then 200mΩ*cm³. The intrinsic resistance of its drift layer estimated from itsthickness and doping was slightly above 60 mΩ*cm³, with the remainder ofthe on-resistance was contributed by the channel regions.

In order to reduce the specific on-resistance of SiC power VJFETs, thesedevices can be driven in bipolar mode by applying high positivegate-to-source voltage. For example, the device discussed above anddisclosed in [13] demonstrated an R_(sp-on) of 66.7 mΩ*cm³ when agate-to-source bias of 5 V was applied [14]. This approach, however, canlead to significant power losses due to high gate current.

Another approach is to use special circuits and methods for controllingnormally-on devices so that they can be operated in normally-off mode. Acascode connection of a low-voltage control JFET with a high-voltageJFET wherein the drain of the control JFET is connected to the source ofthe high-voltage device and the gate of high-voltage JFET is connectedto the source of the control JFET has been disclosed in U.S. Pat. No.3,767,946 [15]. A compound field-effect transistor monolithicallyimplementing such a cascode connection has also been disclosed in U.S.Pat. No. 4,107,725 [16]. Similar types of cascode circuits, wherelow-voltage normally-off devices control high-voltage normally-ondevices are disclosed in U.S. Pat. No. 4,663,547 [17]. More recently, anormally-on SiC VJFET controlled by an Si MOSFET in the aboveconfiguration has been reported by several groups (e.g., [18]). Thisintegrated power switch has demonstrated excellent voltage-blocking andcurrent-conducting capabilities, as well as high switching speed.However, the use of silicon MOSFETs for the control of power innormally-on SiC VJFETs significantly limits both the temperature rangeand the radiation tolerance of the cascode. Accordingly, there is stilla need for wide bandgap normally-off power switching device in general,and in particular, for such a power switch integrated with controlcircuitry built in wide bandgap semiconductors.

SUMMARY

According to a first embodiment, a monolithic integrated circuit isprovided which comprises:

a substrate having opposed first and second major surfaces; and

first and second junction field-effect transistors on discrete locationson the first major surface of the substrate, each of the first andsecond junction field-effect transistors comprising:

a drain layer of an n-type semiconductor material on and non-coextensivewith the first major surface of the substrate such that portions of thesubstrate surrounding the drain layer are exposed;

a drift layer of an n-type semiconductor material on and non-coextensivewith the drain layer such that portions of the drain layer are exposed,the drift layer having a lower conductivity than the drain layer;

one or more raised regions on discrete locations on the drift layer,each raised region comprising a channel region of an n-typesemiconductor material on the drift layer and a source region of ann-type semiconductor material on the channel region, the semiconductormaterial of the source region having a higher conductivity than that ofthe channel region;

a gate region of a p-type semiconductor material on the drift layeradjacent the one or more raised regions and forming a rectifyingjunction with n-type material of the drift layer and the channelregion(s);

ohmic contacts on the gate and source regions and on exposed portions ofthe drain layer;

a first electrical connection between the source ohmic contact of thefirst junction field-effect transistor and the gate ohmic contact of thesecond junction field-effect transistor; and

a second electrical connection between the drain ohmic contact of thefirst junction field-effect transistor and the source ohmic contact ofthe second junction field-effect transistor.

According to a second embodiment, a monolithic integrated circuit isprovided which comprises:

a substrate having opposed first and second major surfaces; and

a buffer layer of a p-type semiconductor material on the first majorsurface of the substrate;

first and second discrete channel regions each of an n-typesemiconductor material in spaced relation on the buffer layer, thesecond channel region comprising a base portion on the buffer layer andan upper portion, the base portion extending laterally beyond the upperportion so as to form a shoulder;

a source region of an n-type semiconductor material on the buffer layeradjacent to and in contact with the first channel region;

a source/drain region of an n-type semiconductor material on the bufferlayer between the first channel region and the second channel region andin contact with both the first channel region and the second channelregion, a portion of the source/drain region overlapping the shoulderportion of the second channel region;

a drain region on the shoulder of the second channel region such thatthe drain region does not directly contact the buffer layer;

a first gate region of a p-type semiconductor material on the firstchannel region and forming a rectifying junction therewith;

a second gate region of a p-type semiconductor material on an uppersurface of the top portion of the second channel region and forming arectifying junction therewith; and

ohmic contacts on the source region, the first and second gate regions,the source/drain region and the drain region.

According to a third embodiment, an integrated circuit is provided whichcomprises:

a first vertical channel JFET comprising:

a substrate having opposed first and second surfaces;

a drain layer of an n-type semiconductor material on the first surfaceof the substrate;

a drift layer of an n-type semiconductor material on and non-coextensivewith the drain layer such that portions of the drain layer are exposed,the drift layer having a lower conductivity than the drain layer;

one or more raised regions comprising a channel region of an n-typesemiconductor material on the drift layer and a source region of ann-type semiconductor material on the channel region, the material of thesource region having a higher conductivity than that of the channelregion;

a gate region of a p-type semiconductor material on the drift layeradjacent the one or more raised regions and forming a rectifyingjunction with the drift layer and the channel region(s);

ohmic contacts the gate and source regions and on exposed portions ofthe drain layer;

a second vertical channel JFET discrete from the first vertical channelJFET comprising:

a substrate of an n-type semiconductor material having opposed first andsecond major surfaces;

a drain layer of an n-type semiconductor material on the first majorsurface of the substrate;

a drift layer of an n-type semiconductor material on the drain layer,the drift layer having a lower conductivity than the drain layer;

one or more raised regions comprising a channel region of an n-typesemiconductor material on the drift layer and a source region of ann-type semiconductor material on the channel region, the material of thesource region having a higher conductivity than that of the channelregion;

a gate region of a p-type semiconductor material on the drift layeradjacent the one or more raised regions and forming a rectifyingjunction with the drift layer and the channel region(s); and

ohmic contacts on the gate and source regions and on the second majorsurface of the substrate;

a first electrical connection between the drain ohmic contact of thefirst vertical channel JFET and the source ohmic contact of the secondvertical channel JFET; and

a second electrical connection between the source ohmic contact of thefirst vertical channel JFET and the gate ohmic contact of the secondvertical channel JFET.

According to a fourth embodiment, an integrated circuit is providedwhich comprises:

a discrete lateral channel JFET comprising:

a substrate having opposed first and second major surfaces;

a buffer layer of a p-type semiconductor material on the first majorsurface of the substrate;

discrete source and drain regions each of an n-type semiconductormaterial in spaced relation on the buffer layer;

a channel region of an n-type semiconductor material on the buffer layerbetween the source and drain regions and in contact with each of thesource and drain regions;

a gate region of a p-type semiconductor material on the channel regionand forming a rectifying junction therewith;

ohmic contacts on the source, gate, and drain regions;

a discrete vertical channel JFET comprising:

a substrate of an n-type semiconductor material having opposed first andsecond major surfaces;

a drain layer of an n-type semiconductor material on the first majorsurface of the substrate;

a drift layer of an n-type semiconductor material on the drain layer,the drift layer having a lower conductivity than the drain layer;

one or more discrete raised regions each comprising a channel region ofan n-type semiconductor material on the drift layer and a source regionof an n-type semiconductor material on the channel region, the materialof the source region having a higher conductivity than that of thechannel region;

a gate region of a p-type semiconductor material on the drift layeradjacent the one or more raised regions and forming a rectifyingjunction with the drift layer and the channel region(s); and

ohmic contacts on the gate and source regions and on the second majorsurface of the substrate;

a first electrical connection between the drain ohmic contact of thelateral channel JFET and the source ohmic contact of the verticalchannel JFET; and

a second electrical connection between the source ohmic contact of thelateral channel JFET and the gate ohmic contact of the vertical channelJFET.

According to a fifth embodiment, a monolithic lateral channel junctionfield-effect transistor (JFET) is provided which comprises:

a substrate having opposed first and second major surfaces; and

a buffer layer of a p-type semiconductor material on the first majorsurface of the substrate;

a channel layer of an n-type semiconductor material on the buffer layer;

discrete source and drain regions of an n-type semiconductor material inspaced relation on the channel layer;

a source/drain region of an n-type semiconductor material on the channellayer between the source and drain regions and spaced from each of thesource and drain regions;

a first gate region of a p-type semiconductor material formed in thechannel layer between the source and source/drain regions and forming arectifying junction with the channel layer;

a second gate region of a p-type semiconductor material formed in thechannel layer between the source/drain and drain regions and forming arectifying junction with the channel layer;

ohmic contacts on the source region, the first and second gate regions,the source/drain region and the drain region.

According to a sixth embodiment, an integrated circuit is provided whichcomprises:

a discrete lateral channel JFET comprising:

a substrate having opposed first and second major surfaces; and

a buffer layer of a p-type semiconductor material on the first surfaceof the substrate;

a channel layer of an n-type semiconductor material on the buffer layer;

discrete source and drain regions of an n-type semiconductor material inspaced relation on the channel layer;

a gate region of a p-type semiconductor material formed in the channellayer between the source and drain regions and forming a rectifyingjunction with the channel layer;

ohmic contacts on the source region, the gate region, and the drainregion;

a discrete vertical channel JFET comprising:

a substrate of an n-type semiconductor material having opposed first andsecond major surfaces;

a drain layer of an n-type semiconductor material on the first majorsurface of the substrate;

a drift layer of an n-type semiconductor material on the drain layer,the drift layer having a lower conductivity than the drain layer;

one or more discrete raised regions each comprising a channel region ofan n-type semiconductor material on the drift layer and a source regionof an n-type semiconductor material on the channel region, the materialof the source region having a higher conductivity than that of thechannel region;

a gate region of a p-type semiconductor material on the drift layeradjacent the one or more raised regions and forming a rectifyingjunction with the drift layer and the channel region(s); and

ohmic contacts on the gate and source regions and on the second majorsurface of the substrate;

a first electrical connection between the source ohmic contact of thelateral channel JFET and the gate ohmic contact of the vertical channelJFET; and

a second electrical connection between the drain ohmic contact of thelateral channel JFET and the source ohmic contact of the verticalchannel JFET.

According to a seventh embodiment, a monolithic integrated circuit isprovided which comprises a lateral junction field effect transistor anda vertical junction field effect transistor;

the lateral junction field effect transistor comprising:

a buffer layer of a p-type semiconductor material formed in a portion ofa first major surface of a drift layer;

a channel layer of an n-type semiconductor material on andnon-coextensive with the buffer layer such that a portion of the bufferlayer is exposed;

discrete source and drain regions of an n-type semiconductor material inspaced relation on the channel layer;

a gate region of a p-type semiconductor material formed in the channellayer between the source and drain regions and forming a rectifyingjunction with the channel layer;

ohmic contacts on the source region, the gate region, the drain regionand on the exposed portion of the buffer layer; the vertical junctionfield effect transistor comprising:

a channel layer of an n-type semiconductor material on the first majorsurface of the drift layer laterally spaced from the buffer layer;

one or more discrete source regions of an n-type semiconductor materialin spaced relation on the channel layer;

a gate region of a p-type semiconductor material formed in the channellayer adjacent the one or more source regions and forming a rectifyingjunction with the channel layer; and

ohmic contacts on the gate and source regions;

wherein the drift layer is on a drain layer of an n-type semiconductormaterial which is on a first major surface of a substrate; and whereinan electrical contact is on a second major surface of the substrateopposite the first major surface of the substrate.

According to an eighth embodiment, a monolithic integrated circuit isprovided which comprises a lateral junction field effect transistor anda vertical junction field effect transistor;

the lateral junction field effect transistor comprising:

a buffer layer of a p-type semiconductor material formed in a portion ofa first major surface of a drift layer;

a channel layer of an n-type semiconductor material on andnon-coextensive with the buffer layer such that a portion of the bufferlayer is exposed;

discrete source and drain regions each of an n-type semiconductormaterial in spaced relation on the channel layer;

a metal layer on the channel layer between the source and drain regionsforming a metal-semiconductor rectifying junction with the channellayer;

ohmic contacts on the source region, the drain region and on the exposedportion of the buffer layer;

the vertical junction field effect transistor comprising:

one or more raised regions on the first major surface of the drift layerlaterally spaced from the buffer layer each comprising a channel regionof an n-type semiconductor material on the first major surface of thedrift layer and spaced from the buffer layer of the lateral junctionfield effect transistor and a source region of an n-type semiconductormaterial on the channel region;

a metal layer on the drift layer adjacent to the one or more raisedregions forming a metal-semiconductor rectifying junction with the driftlayer and the channel region(s); and

an ohmic contact on the source region;

wherein the drift layer is on a layer of n-type semiconductor materialwhich is on a first major surface of a substrate; and wherein anelectrical contact is on a second major surface of the substrateopposite the first major surface of the substrate.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic cross-section of a monolithic inverter circuitcomprising enhanced and depletion mode LTJFETs.

FIG. 2 is a schematic cross-section of a monolithic normally-off JFETcomprising enhanced and depletion mode LTJFETs having a built-in PiNdiode.

FIGS. 3A and 3B are a circuit representation (FIG. 3A) and an examplelayout (FIG. 3B) of a monolithic normally-off JFET integrated circuitcomprising enhanced and depletion mode LTJFETs having a built-in PiNdiode.

FIG. 4 is a schematic cross-sectional representation of a monolithicnormally-off JFET built using enhanced and depletion mode LTJFETsintegrated with an SBD or a JBS diode.

FIGS. 5A and 5B are a circuit representation (FIG. 5A) and an examplelayout (FIG. 5B) of a monolithic normally-off JFET integrated circuitcomprising enhanced and depletion mode LTJFETs integrated with an SBD ora JBS diode.

FIG. 6 is a schematic cross-sectional representation of a hybridnormally-off JFET built using an enhanced mode LTJFET and a depletionmode VJFET having a built-in PiN diode.

FIG. 7 is a schematic cross-sectional representation of a hybridnormally-off JFET built using enhanced mode LTJFETs and a depletion modeVJFET integrated with an SBD or a JBS diode.

FIG. 8 is a circuit representation of a monolithic LTJFET timer circuitdriving a built-on-chip low-voltage high-current enhanced-mode LTJFETconnected in cascode with a discrete high-voltage normally-on powerVJFET.

FIG. 9 is a schematic cross-sectional representation of a monolithicinverter circuit built using enhanced and depletion mode overgrown-gateLJFETs.

FIG. 10 is a schematic cross-sectional representation of a hybridnormally-off JFET comprising an enhanced mode overgrown-gate LJFET and adepletion mode VJFET.

FIG. 11 is a schematic cross-sectional representation of a hybridnormally-off JFET power-switch comprising a low voltage enhanced modeLJFET and a high voltage discrete normally-on depletion mode VJFET.

FIG. 12 is a schematic cross-sectional representation of a monolithicinverter circuit built using enhanced and depletion mode implanted-gateLJFETs.

FIG. 13 is a schematic cross-sectional representation of a monolithicnormally-off JFET integrated circuit built using enhanced and depletionmode implanted-gate LJFETs.

FIG. 14 is a schematic cross-sectional representation of hybridnormally-off JFET integrated circuit built using an enhanced modeimplanted-gate LJFET and a depletion mode VJFET.

FIG. 15 is a schematic cross-sectional representation of a hybridnormally-off JFET integrated circuit built using an enhanced modedual-gate LJFET and a depletion mode VJFET wherein the bottom gate ofthe LJFET is implanted into the drift region.

FIG. 16 is a schematic cross-sectional representation of a hybridguard-ring terminated normally-off JFET integrated circuit built usingan enhanced mode dual-gate LJFET and a depletion mode VJFET wherein thebottom gate of the LJFET and the guard rings are implanted into thedrift region.

FIG. 17 is a schematic cross-sectional representation of a hybridguard-ring terminated normally-off JFET built using an enhanced modedual-gate LJFET and a depletion mode VJFET with a Schottky gate whereinthe bottom gate of the LJFET and the guard rings are implanted into thedrift region.

FIGS. 18A-18D are a simulated device structure (FIG. 18A), schematiccross sectional representation (FIG. 18B) and graphs showing the outputDC characteristics (FIGS. 18C and 18D) of a SiC LTJFET integratedswitch.

FIGS. 19A-19D are a photograph (FIG. 19A), circuit representation (FIG.19B) and graphs (FIGS. 19C and 19D) showing measured characteristics ofa hybrid normally-off 900 V power switch.

FIGS. 20A and 20B are a circuit representation (FIG. 20A) and a graph(FIG. 20B) showing measured internal voltages of a hybrid normally-off,900 V power switch.

FIGS. 21A-21C are schematic representations of distributed drainresistances of an LTJFET (FIG. 21A) and a VJFET (FIG. 21B) along with agraph (FIG. 21C) showing the resistance of the lateral drain layer of anLTJFET normalized to the resistance of the vertical drain of a VJFET asa function of finger length for different doping levels of the lateraldrain layer.

FIGS. 22A-22H illustrate a method of making a monolithic integratedcircuit as set forth in FIGS. 9 and 10.

FIGS. 23A-23H illustrate a method of making a monolithic integratedcircuit as set forth in FIGS. 12 and 13.

FIGS. 24A-24J illustrate a method of making a monolithic integratedcircuit as set forth in FIG. 15.

FIGS. 25A-25D illustrate a method of making a monolithic integratedcircuit as set forth in FIG. 17.

REFERENCE NUMERALS

The reference numerals used in the drawings are defined as set forthbelow. For the substrate, implanted regions, and epitaxially grownlayers, representative thicknesses and doping concentrations are alsoprovided.

# Material 1 Substrate (e.g., semi-insulating substrate) 1a N-typesubstrate (e.g., doping level >1 × 10¹⁸ cm⁻³) 2 Epitaxially grown layer(p-type) (e.g., ≧0.1 μm thick, 1 × 10¹⁵-1 × 10¹⁷ cm⁻³) 3 Epitaxiallygrown layer (n-type) (e.g., 0.2-5 μm, >5 × 10¹⁸ cm⁻³) 3a Epitaxiallygrown layer (n-type) (e.g., 0.5-1 μm, >5 × 10¹⁸ cm⁻³) 4 Epitaxiallygrown layer (n-type) (e.g., 0.5-10 μm 5 × 10¹⁵- 5 × 10¹⁷ cm⁻³) 4aEpitaxially grown layer (n-type) (e.g., 5-350 μm, 2 × 10¹⁴- 2 × 10¹⁶cm⁻³) 5 Epitaxially grown layer (n-type) (e.g., 0.2-1.5 μm, 5 × 10¹⁵- 5× 10¹⁷ cm⁻³ 5a Epitaxially grown layer (n-type) (e.g., 0.2-1.5 μm, 5 ×10¹⁵- 2 × 10¹⁷ cm⁻³) 5b Epitaxially grown layer (n-type) (e.g., 0.3-1.5μm, 5 × 10¹⁵- 2 × 10¹⁷ cm⁻³) 6 Epitaxially grown layer (n-type) (e.g.,0.2-1.5 μm, >5 × 10¹⁸ cm⁻³) 6a Implanted region (n-type) (e.g., ≧0.1 μm,≧5 × 10¹⁸ cm⁻³) 7 Implanted region (p-type) (e.g., ≧0.1 μm ≧5 × 10¹⁸cm⁻³) 7a Epitaxially grown layer (p-type) (e.g., 0.2-1.5 μm, >5 × 10¹⁸cm⁻³) 8 Ohmic contact 9 Schottky contact

DETAILED DESCRIPTION

The present invention will be described in more detail hereafter withreference to the accompanying drawings and photographs, in whichpreferred embodiments of the invention are described with siliconcarbide (SiC) semiconductor serving as an example.

Silicon carbide crystallizes in numerous (i.e., more than 200) differentmodifications (polytypes). The most important are: 3C—SiC (cubic unitcell, zincblende); 2H—SiC; 4H—SiC; 6H—SiC (hexagonal unit cell,wurtzile); 15R—SiC (rhombohedral unit cell). The 4H polytype is moreattractive for power devices, because of its higher electron mobility.Although the 4H—SiC is preferred, it is to be understood that thepresent invention is applicable to devices and integrated circuitsdescribed herein made of other wide bandgap semiconductor materials suchas gallium nitride, and other polytypes of silicon carbide, by way ofexample.

FIG. 1 shows a schematic cross-section of enhanced and depletion modesemiconductor devices referred to a Lateral Trench Junction Field-EffectTransistors (LTJFETs), and a schematic presentation of electricalconnections used to form a monolithic inverter circuit. As shown, thedevices used to form the inverter are built on a wide bandgapsemiconductor substrate (1), which can be either: semi-insulating;p-type; or n-type with a p-type buffer layer. As shown in FIG. 1, thedevices comprise drain (3), drift (4), channel (5), and source (6)expitaxially grown n-type layers, and p-type implanted gate regions (7).The device structures can be defined using plasma etching and ionimplantation. In the circuit shown in FIG. 1, the ohmic contacts to thesource, gate, and drain regions can be formed on the same side of thewafer, which allows for the devices to be used in monolithic integratedcircuits. A complete description of a device as described above andshown in FIG. 1 as well as an exemplary fabrication method for thisdevice can be found in U.S. patent application Ser. No. 10/999,954entitled: “Lateral Trench Field-Effect Transistors in Wide BandgapSemiconductor Materials, methods of Making, and Integrated CircuitsIncorporating the Transistors”, filed on even date herewith, whichapplication is incorporated by reference herein in its entirety.

FIG. 2 is a schematic representation of a monolithic normally-off JFETcomprising single-finger enhanced and depletion mode LTJFETs and havinga built-in PiN diode. A schematic presentation of electrical connectionsis also shown in FIG. 2. As shown in FIG. 2, the devices are connectedin cascode configuration in such a way that the drain of the enhancedmode transistor (referred to as an “EJFET”) is connected to the sourceof the depletion mode transistor (referred to as a “DJFET”), and thegate of the DJFET is connected to the source of the control EJFET. Thep-n junctions formed in between the gate regions (7) and the drift layer(4) of the DJFET of this device form a so called anti-parallelfree-wheeling PiN diode. The size of this diode can be defined by thewidths of implanted gate regions.

Although FIG. 2 shows single-finger device implementation of anormally-off JFET, in practice multi-finger LTJFETs can be used to formpower switches. FIGS. 3A and 3B shows a schematic circuit representation(FIG. 3A) and an exemplary layout design (FIG. 3B) of a monolithicmulti-finger normally-off power switch.

In order to reduce switching losses, the PiN diode shown as in FIGS. 3Aand 3B can be replaced with a Schottky Barrier diode (SBD) or a JunctionBarrier Schottky (JBS) diode. Methods of forming Schottky gates in atrench structure are disclosed in U.S. Patent Application No.60/585,881, filed Jul. 8, 2004, which application is incorporated byreference herein in its entirety. FIG. 4 provides a schematiccross-section of a monolithic normally-off JFET power switch with anintegrated free-wheeling SBD or JBS diode, and FIGS. 5A and 5B provide aschematic circuit representation (FIG. 5A) and exemplary layout design(FIG. 5B) of such a switch monolithically formed using multi-fingerLTJFETs.

FIGS. 6 and 7 are schematic cross-sectional representations ofsingle-finger normally-off JFET power switches where enhancement-modelow-voltage LTJFETs control high-voltage discrete normally-ondepletion-mode VJFETs. FIG. 6 shows a hybrid JFET power switch with abuilt-in anti-parallel PiN diode, and FIG. 7 shows a JFET power switchcomprising an anti-parallel SBD or JBS diode monolithically integratedwith a high-voltage VJFET.

An exemplary implementation of the technology described above is shownin FIG. 8. As shown in FIG. 8, a monolithic LTJFET timer circuit drivesa built-on-chip low-voltage high-current enhanced-mode LTJFET connectedin cascode with a discrete high-voltage normally-on power VJFET.

Although vertical channel multi-finger LTJFETs are preferable inhigh-power application because of their low specific on-resistance andabsence of trapping effects common in wide bandgap semiconductors,alternative JFET structures (e.g., those with a lateral channel) canalso be employed to form normally-off power JFET switches. FIGS. 9-17illustrate various exemplary embodiments of integrated JFET switchesbuilt using enhanced and depletion mode Lateral Junction Field-EffectTransistors (LJFETs).

FIG. 9 is a schematic cross-sectional representation with electricalconnections of a lateral channel JFET integrated circuit comprisingenhanced and depletion mode LJFETs having expitaxially overgrown gates.As shown in FIG. 9, the integrated circuit forms a monolithic invertercircuit. The LJFETs used to form the inverter are built on the widebandgap semiconductor substrate (1), which can be either:semi-insulating; p-type; or n-type with a p-type buffer layer. As shownin FIG. 9, the integrated circuit comprises buffer (2) and channel (5 a)epitaxially grown n-type layers, as well as implanted source and drain(6 a) regions and expitaxially grown p-type gate regions (7 a). Thedevice structures can be defined using plasma etch and ion implantation.The ohmic contacts (8) to the source, gate, and drain regions can beformed on the same side of the wafer allowing for the use of the devicein monolithic integrated circuits.

FIG. 10 is a schematic representation of a pitch of a monolithicnormally-off JFET power switch built using enhanced and depletion modeLJFETs with overgrown gate regions. As can be seen from the schematicpresentation of electrical connections, the devices are connected incascode configuration in such a way that the drain of the low-voltageenhanced mode LJFET (referred to as an “ELJFET”) is connected to thesource of the higher-voltage depletion mode LJFET (referred to as a“DLJFET”), and the gate of the DLJFET is connected to the source of thecontrol ELJFET.

FIG. 11 shows a schematic cross-section of a hybrid normally-off JFETpower switch wherein a low-voltage ELJFET controls a high-voltagediscrete normally-on depletion-mode VJFET.

An alternative LJFET structure where source and drain regions are formedin an epitaxially grown n-type layer and gate regions are defined by ionimplantation can also be used. Devices of this type are shown in FIGS.12-17.

FIG. 12 shows is a schematic cross-sectional representation withelectrical connections of a monolithic inverter, circuit comprisingenhanced and depletion mode implanted-gate LJFETs. As shown, the devicesused to form the inverter are built on a wide bandgap semiconductorsubstrate (1), which can be either: semi-insulating; p-type; or n-typewith a p-type buffer layer. As also shown, the device comprises buffer(2), channel (5 b), source and drain (6) epitaxially grown n-typelayers, as well as implanted gate (7) regions.

FIG. 13 is a schematic cross-sectional representation of a pitch of amonolithic normally-off JFET power switch built using enhanced anddepletion mode implanted-gate LJFETs. As shown in FIG. 13, the drain ofthe D-mode LJFET is laterally spaced from the gate on the channel layer(5 b) to form a lateral drift region in the device.

FIG. 14 is a schematic cross-sectional representation of a normally-offJFET power switch where an enhancement-mode low-voltage implanted-gateLJFET controls a high-voltage discrete normally-on depletion-mode VJFET.

FIG. 15 is a schematic cross-sectional representation of a monolithicnormally-off JFET power switch wherein an enhancement-mode low-voltagedual-gate LJFET controls a high-voltage discrete normally-ondepletion-mode VJFET. As shown in FIG. 15, the bottom gate of the LJFETis implanted into drift region (4) before the channel region is grownthereon.

FIG. 16 is a schematic cross-sectional representation of a device asshown in FIG. 3D wherein the bottom gate of the LJFET is implanted intodrift region 4 together with guard rings. The guard rings can be used toincrease the voltage blocking capability of the switch.

Although FET devices having implanted p-type gates are described above,Schottky gates can also be employed for the fabrication of anormally-off FET power switch. FIG. 17 is a schematic cross-sectionalrepresentation of a device as shown in FIG. 16 wherein the implantedp-type top gate of the LJFET and the implanted gate of the discretenormally-on depletion-mode VJFET are replaced with Schottky gates. Asshown, the Schottky gate of the discrete normally-on FET also serves asan integrated anti-parallel free-wheeling diode.

FIGS. 18A-18D shows a simulated device structure (FIG. 18A), schematiccross-sectional representation (FIG. 18B) and graphs showing the outputDC characteristics (FIGS. 18C and 18D) of a SiC LTJFET integratedswitch, where both the EJFET and the DJFET have channel peripheries of 1cm.

In order to demonstrate feasibility of the above described cascode powerswitch, a hybrid embodiment of the switch was constructed using discretenon-terminated enhanced and depletion mode vetical JFETs. FIGS. 19A-19Dare a photograph (FIG. 19A), a schematic representation (FIG. 19B) andgraphs showing measured characteristics (FIGS. 19C and 19D) of a hybridnormally-off, 900 V power switch. As can be seen from FIGS. 19C and 19D,despite relatively high leakage current (I_(D)=330 μA @ V_(DS)=900 V andV_(GS)=0 V) induced by the depletion mode device, the voltage-controlledSiC power switch was controlled by as little as 2.75 V.

The basic function of the switch can be described as follows. At theHIGH control level (e.g., V_(GS)=2.75 V), the enhanced mode transistor(EJFET) is turned on. Between the gate and source of the depletion modetransistor (DJFET) only a small voltage drop occurs, therefore, DJFET ison too. If EJFET is turned off with the LOW control level (VGS=0.25 V)its drain-to-source voltage increases to 40-50V as shown in FIG. 20B.This voltage pinches-off the DJFET.

The specific on-resistance of the integrated switch can be minimized asfollows. First, the ratios of pinch-off voltages and channel peripheriesof both transistors (e.g., EJFET and DJFET) can be adjusted so that theywill have approximately equal on-resistances and neither one willtherefore limit the overall current. Second, the device can beconstructed such that the gate-to-source breakdown voltage of DJFET isequal or higher than the drain-to-source breakdown voltage of EJFET.

In addition, the finger length of high-current multi-finger LTJFETs canbe reduced to keep the resistances of the alteral drain regioncompatible to the resistance of the vertical n⁺ substrate. FIGS. 21A and21B are schematic representations of distributed drain resistances ofLTJFET (FIG. 21A) and VJFET (FIG. 21B), and graph (FIG. 21C) showingresistance of the lateral drain layer of LTJFET normalized to theresistance of the vertical drain of VJFET as a function of finger lengthfor different dopings of the lateral drain layer. As can be seen fromFIG. 21C, for a heavily doped 1-μm thick lateral drain layer (3), thefinger length of the LTJFET will preferably not exceed 100 μm in length.The finger length, however, can be increased by increasing the thicknessand/or the doping levels of the drain layer (3).

FIGS. 22A-22H illustrate a method of making a device as set forth inFIG. 9. FIG. 22A shows a multi-layer structure comprising a substrate(1), an epitaxially grown p-type layer (2), and an epitaxially grownn-type layer (5 a). An etch mask (10) is positioned on the exposedsurface of epitaxially grown n-type layer (5 a) as shown in FIG. 22B.Epitaxially grown n-type layer (5 a) is then selectively etched (12) asshown in FIG. 22B. Etch mask (10) is then removed and ion implantationmask (14) is then placed on the etched surface of epitaxially grownn-type layer (5 a) as shown in FIG. 22D. Ion implantation of n-typedopants through mask (14) results in the formation of highly n-dopedregions (6 a) in epitaxially grown n-type layer (5 a) as shown in FIG.22E. Mask (14) is then removed and a layer of p-type semiconductormaterial (7 a) is grown on the etched and implanted surface ofepitaxially grown n-type layer (5 a) as shown in FIG. 22F. Etch mask(16) is then positioned on the exposed surface of layer (7 a) as shownin FIG. 22G. Etching through mask (16) results in selective removal oflayer (7 a) and formation of raised p-type features as also shown inFIG. 22G. Finally, mask (16) is removed and ohmic contacts are formed onexposed surfaces of the raised p-type features and the implanted regions(6 a).

The method as outlined above can also be used, by selecting appropriatemasks, to form a structure as shown in FIG. 10.

FIGS. 23A-23H illustrate a method of making a structure as shown in FIG.12. FIG. 23A shows a substrate (1), an epitaxially grown p-type layer(2) on the substrate (1), and an epitaxially grown n-type layer (5 b) onlayer (2). As shown in FIG. 23B, an etch mask (18) is positioned on theexposed surface of layer (5 b). Etching (20) results in selectiveremoval of material from layer (5 b) as shown in FIG. 23C. After removalof mask (18), an n-type epitaxial layer (6) is grown on the etchedsurface of layer (5 b) as shown in FIG. 23D. Etch mask (22) ispositioned on the exposed surface of layer (6) as shown in FIG. 23E andetching (24) results in selective removal of material from layer (6) andexposure of underlying layer (5 b) as shown in FIG. 23F. Mask (22) isthen used to selectively implant p-type donors in exposed surface oflayer (5 b) to form implanted gate regions (7) as shown in FIG. 23G.Ohmic contacts (8) are then formed on the implanted p-type gate regions(7) to form the gate contacts and on the raised n-type regions (6) toform the source and drain contacts for the device as shown in FIG. 23H.

The method as outlined above can also be used, by selecting appropriatemasks, to form a structure as shown in FIG. 13.

FIGS. 24A-24J illustrate a method of making a structure as shown in FIG.15. FIG. 24A shows an n-type substrate (1 a), an epitaxially grownn-type layer (3 a) on substrate (1 a), and an epitaxially grown n-typelayer (4 a) on layer (3 a). An ion implantation mask (26) is also shownon the exposed upper surface of layer (4 a). As shown in FIG. 24B, layer(4 a) is selectively implanted with p-type donor atoms through mask (26)to form gate region (7). After removal of mask (26), an n-type epitaxiallayer (5) and an n-type epitaxial layer (6) are successively grown onthe implanted surface of layer (4 a) as shown in FIGS. 24C and 24D. Etchmask (30) is then positioned on the exposed surface of layer (6) asshown in FIG. 24D followed by etching (31) through layer (6) andpartially through underlying layer (5) (FIG. 24E). Exposed portions oflayer (5) are then implanted with p-type donor atoms through mask (30)to form additional gate regions (7) as shown in FIG. 24F. Etch mask (34)is then positioned on the surface of the etched and implanted structureand etching (36) results in selective removal of portions of layer (5)including portions of the p-type implanted gate regions (FIG. 24H).Exposed portions of layer (4 a) are then etched (40) through mask (38)as shown in FIG. 24I. Ohmic contacts (8) are then formed on the etchedand implanted structure to form the device as shown in FIG. 24J.

The method as outlined above can also be used to form a structure asshown in FIG. 16.

FIGS. 25A-25D illustrate a method of making a structure as shown in FIG.17. As shown in FIG. 25A, a structure a shown in FIG. 24E is etched (44)through mask (42) to expose portions of underlying layer (4 a) (FIG.25B). Schottky contacts (9) are then formed on the etched/implantedstructure as shown in FIG. 25C. The formation of ohmic contacts (8)results in the device as shown in FIG. 25D.

Although exemplary embodiments are discussed above, other alternativeembodiments are also possible. For example, GaN n-type epitaxial layerscan be grown on silicon carbide, sapphire, or silicon substrates to forma starting material stack for the fabrication of the proposed devicestructure. Alternatively, a substrate material comprising a conductingSiC substrate with a semi-insulating epitaxially grown buffer layer canbe used as disclosed in U.S. patent application Ser. No. 10/033,785,filed Jan. 3, 2002 (published as U.S. Patent Publication No.2002-0149021).

The SiC layers can be formed by doping the layers with donor or acceptormaterials using known techniques. Exemplary donor materials includenitrogen and phosphorus. Nitrogen is a preferred donor material.Exemplary acceptor materials for doping SiC include boron and aluminum.Aluminum is preferred acceptor material. The above materials are merelyexemplary, however, and any acceptor and donor materials which can bedoped into silicon carbide can be used. The doping levels andthicknesses of the various layers of the LTJFETs, LJFETs and VJFETsdescribed herein can be varied to produce a device having desiredcharacteristics for a particular application. Similarly, the dimensionsof the various features of the device can also be varied to produce adevice having desired characteristics for a particular application.

The SiC layers can be formed by epitaxial growth on a suitablesubstrate. The layers can be doped during epitaxial growth.

While the foregoing specifications teach the principles of the presentinvention, with examples provided for the purpose of illustration, itwill be appreciated by one skilled in the art from reading thisdisclosure that various changes in form and detail can be made withoutdeparting from the true scope of the invention.

CITED REFERENCES

1. W. Xie et al., “Monolithic NMOS Digital Integrated Circuits in6H—SiC,” IEEE Electron Device Letters, Vol.: 15, No.: 11, Nov. 11, 1994,pp. 455-457.

2. D. M. Brown et al. “High temperature silicon carbide planar ICtechnology and first monolithic SiC operational amplifier IC,”Transactions of 2^(nd) Int. High-Temp. Elec. Conf. (HiTEC), 1994, pp.XI-17-XI-22.

3. Slater, Jr. et al., “Silicon Carbide CMOS devices,” U.S. Pat. No.6,344,663, Feb. 5, 2002.

4. M. Bhatnagar et al., “Lateral MOSFET with modified field plates anddamage areas,” U.S. Pat. No. 5,710,455, Jan. 20, 1998.

5. I. Sankin et al., “On development of 6H—SiC LDMOS transistors usingsilane-ambient implant anneal,” Solid-State Electronics, Vol. 45, No. 9,September, 2001, pp. 1653-165.

6. S. T. Sheppard et al., “High power hybrid and MMIC amplifiers usingwide-bandgap semiconductor devices on semi-insulating SiC substrates,”Digest of 60th Device Research Conference, 2002, Jun. 24-26, 2002, pp.:175-178.

7. M. P. Lam, “Ion implant technology for 6H—SiC MESFETs digital ICs,”Digest of 54th Annual Device Research Conference, 1996., Jun. 24-26,1996, pp. 158-159.

8. Casady et al., “Complementary accumulation-mode JFET integratedcircuit topology using wide (>2 eV) bandgap semiconductors,” U.S. Pat.No. 6,503,782, Jan. 7, 2003.

9. J. N. Merrett et al., “Silicon Carbide Vertical Junction Field EffectTransistors Operated at Junction Temperatures Exceeding 300° C.”,Proceedings of IMAPS International Conference and Exhibition on HighTemperature Electronics (HiTECH 2004), May 17-20, 2004, Sante Fe, N.Mex.

10. Sugawara et al., “Vertical field-effect semiconductor device withburied gate region,” U.S. Pat. No. 6,600,192, Jul. 29, 2003.

11. Friedrichs et al., “Semiconductor construction with buried islandregion and contact region,” U.S. Pat. No. 6,693,322, Feb. 17, 2004.

12. J. H. Zhao, “Double-gated vertical junction field effect powertransistor,” U.S. Published Patent Application 20030089930, May 15,2003.

13. K. Asano et al., “5.5 kV normally-off low RonS 4H—SiC SEJFET,” PowerSemiconductor Devices and ICs, 2001. ISPSD '01. Proceedings of the 13thInternational Symposium on, 4-7 Jun. 2001, pp. 23-26.

14. Y. Sugawara et al., “4H—SiC high power SIJFET module,” PowerSemiconductor Devices and ICs, 2003. Proceedings, ISPSD '03. 2003 IEEE15th International Symposium on, 14-17 Apr. 2003, pp. 127-130.

15. Berger et al., “Junction Field Effect Transistor Device forReplacing a Pentode,” U.S. Pat. No. 3,767,946, Oct. 23, 1973.

16. Yoshida et al., “Compound Field Effect Transistor,” U.S. Pat. No.4,107,725, Aug. 15, 1978.

17. Baliga et al., “Composite Circuit for Power SemiconductorSwitching”, U.S. Pat. No. 4,663,547, May 5, 1987.

18. P. Friedrichs et al., “SiC power devices with low on-resistance forfast switching applications,” Power Semiconductor Devices and ICs, 2000,Proceedings of the 12th International Symposium, May 22-25, 2000, pp.213-216.

1. A method comprising: positioning a first mask on a layer of n-typesemiconductor material, wherein the layer of n-type semiconductormaterial is on a first layer of p-type semiconductor material and thefirst layer of p-type semiconductor material is on a substrate;selectively etching the layer of n-type semiconductor material throughopenings in the first mask to form an etched region and a raised regionhaving a sidewall adjacent the etched region; removing the first mask;positioning a second mask on the layer of n-type semiconductor materialwhich masks a portion of the etched region and a portion of the raisedregion; implanting n-type dopants in the layer of n-type semiconductormaterial through openings in the mask to form a first non-implantedregion over the etched region, a second non-implanted region over theraised region and n-type implanted regions in the layer of n-typesemiconductor material; removing the second mask; epitaxially growing asecond layer of p-type semiconductor material on the etched andimplanted layer of n-type semiconductor material; positioning a thirdmask on the second layer of p-type semiconductor material which masks aportion of the second layer of p-type semiconductor material over thefirst non-implanted region and a portion of the second layer of p-typesemiconductor material over the second non-implanted region; using thethird mask, selectively etching through the second layer of p-typesemiconductor material to expose implanted regions in the underlyinglayer of n-type semiconductor material thereby forming raised featuresof p-type semiconductor material; removing the third mask; and formingohmic contacts on the raised features and on the implanted regions inthe layer of n-type semiconductor material.
 2. The method of claim 1,wherein the n-type implanted regions have a dopant concentration of5×10¹⁸ cm⁻³ or greater and a thickness of 0.1 μm or greater.
 3. Themethod of claim 1, wherein the substrate is a semi-insulating substrate.4. The method of claim 1, wherein the layer of n-type semiconductormaterial has a thickness of 0.2 to 1.5 μm and a dopant concentration,prior to implantation, of 5×10¹⁵ to 2×10¹⁷ cm⁻³.
 5. The method of claim1, wherein the second layer of p-type semiconductor material isepitaxially grown to a thickness of 0.2 to 1.5 μm and has a dopingconcentration greater than 5×10¹⁸ cm⁻³.
 6. The method of claim 1,wherein the first layer of p-type semiconductor material has a thicknessof 0.1 μm or greater and a dopant concentration of 1×10¹⁵ to 1×10¹⁷cm⁻³.
 7. A method comprising: positioning a first mask on a first layerof n-type semiconductor material, wherein the first layer of n-typesemiconductor material is on a layer of p-type semiconductor materialand the layer of p-type semiconductor material is on a substrate;selectively etching the first layer of n-type semiconductor materialthrough openings in the first mask to form an etched region and a raisedregion; removing the first mask; epitaxially growing a second layer ofn-type semiconductor material on the etched and implanted layer ofn-type semiconductor material; positioning a second mask on the secondlayer of n-type semiconductor material such that openings in the secondmask are located over the etched region of the layer of n-typesemiconductor material and over the raised region of the layer of n-typesemiconductor material; using the second mask, selectively etchingthrough the second layer of n-type semiconductor material to expose theunderlying first layer of n-type semiconductor material and to formraised features of n-type semiconductor material; selectively implantingp-type dopants in the first layer of n-type semiconductor materialthrough the openings in the second mask to form p-type implantedregions; removing the second mask; and forming ohmic contacts on exposedsurfaces of the raised features of n-type semiconductor material and thep-type implanted regions.
 8. The method of claim 7, wherein the secondlayer of n-type semiconductor material is more heavily doped that thefirst layer of n-type semiconductor material.
 9. The method of claim 7,wherein the second layer of n-type semiconductor material has athickness of 0.2 to 1.5 μm and a dopant concentration greater than5×10¹⁸cm⁻³.
 10. The method of claim 7, wherein the first layer of n-typesemiconductor material has a thickness of 0.3 to 1.5 μm and a dopantconcentration of 5×10¹⁵to 2×10¹⁷cm⁻³.
 11. The method of claim 7, whereinthe p-type implanted regions have a thickness of 0.1 μm or greater and adopant concentration greater than 5×10¹⁸cm⁻³.
 12. A method comprising:positioning a first mask on a first layer of n-type semiconductormaterial, wherein the first layer is on a second layer of n-typesemiconductor material and the second layer is on a substrate; using thefirst mask, selectively implanting p-type dopants in the first layer toform a p-type implanted region adjacent a non-implanted region in thefirst layer; removing the first mask; epitaxially growing a third layerof n-type semiconductor material on the first layer; epitaxially growinga fourth layer of n-type semiconductor type material on the third layer;positioning a second mask on the fourth layer; selectively etchingthrough the fourth layer to expose underlying third layer throughopenings in the second mask thereby forming raised features of n-typesemiconductor material over the p-type implanted region of the firstlayer and one or more raised features of n-type semiconductor materialover the non-implanted region of the first layer; implanting p-typedopants in the third layer through openings in the second mask to formp-type implanted regions in the third layer between and adjacent theraised features of n-type semiconductor material; removing the secondmask; positioning a third mask which masks the raised features and thearea between the raised features over the p-type implanted region of thefirst layer and which masks the one or more raised features over thenon-implanted region of the first layer and areas adjacent thereto;using the third mask, selectively etching through the third layer toexpose p-type implanted and non-implanted regions of the underlyingfirst layer thereby forming first and second raised structures, whereinthe first raised structure comprises the raised features over the p-typeimplanted region of the first layer and the p-type implanted region ofthe third layer therebetween and wherein the second raised structurecomprises the one or more raised features over the non-implanted regionof the first layer and p-type implanted regions of the third layeradjacent thereto; removing the third mask; positioning a fourth maskcovering the first and second raised structures and a region of p-typeimplanted first layer adjacent the first raised structure; using thefourth mask, selectively etching through the p-type implanted region inthe first layer of n-type semiconductor material adjacent to and betweenthe first and second raised structures; removing the fourth mask formingohmic contacts on exposed surfaces of the raised features of n-typesemiconductor material and on exposed p-type implanted regions.
 13. Themethod of claim 12, wherein the second layer of n-type semiconductormaterial is more heavily doped that the first layer of n-typesemiconductor material.
 14. The method of claim 12, wherein the secondlayer of n-type semiconductor material has a dopant concentrationgreater than 5×10¹⁸ cm⁻³ and a thickness of 0.5 to 1 μm.
 15. The methodof claim 12, wherein the first layer of n-type semiconductor materialhas a dopant concentration of 2×10¹⁴ to 2×10¹⁶ cm⁻³ and a thickness of 5to 350 μm.
 16. The method of claim 12, wherein the p-type implantedregion in the first layer has a thickness of 0.1 μm or greater and adopant concentration or 5×10¹⁸ cm⁻³ or greater.
 17. The method of claim12, wherein the p-type implanted regions in the third layer have athickness of 0.1 μm or greater and a dopant concentration or 5×10¹⁸ cm⁻³or greater.
 18. The method of claim 12, wherein the third layer ofn-type semiconductor material has a dopant concentration of 5×10¹⁵ to5×10¹⁷ cm⁻³ and a thickness of 0.2 to 1.5 μm.
 19. The method of claim12, wherein the fourth layer of n-type semiconductor material has adopant concentration greater than 5×10¹⁸ cm⁻³ and a thickness of 0.2 to1.5 μm.
 20. The method of claim 12, wherein the n-type substrate has adopant concentration greater than 1×10¹⁸ cm⁻³.
 21. A method comprising:positioning a first mask on a first layer of n-type semiconductormaterial, wherein the first layer is on a second layer of n-typesemiconductor material and the second layer is on a substrate; using thefirst mask, selectively implanting p-type dopants in the first layer ofn-type semiconductor material to form a p-type implanted region adjacenta non-implanted region in the first layer; removing the first mask;epitaxially growing a third layer of n-type semiconductor material onthe first layer; epitaxially growing a fourth layer of n-typesemiconductor type material on the third layer; positioning a secondmask on the fourth layer; selectively etching through the fourth layerto expose underlying third layer through openings in the second maskthereby forming raised features of n-type semiconductor material overthe p-type implanted region of the first layer and one or more raisedfeatures of n-type semiconductor material over the non-implanted regionof the first layer; removing the second mask; positioning a third maskwhich masks the raised features and the area between the raised featuresover the p-type implanted region of the first layer and which masks theraised feature over the non-implanted region of the first layer; usingthe third mask, selectively etching through the third layer to exposep-type implanted and non-implanted regions of the underlying first layerthereby forming first and second raised structures, the first raisedstructure comprising the raised features over the p-type implantedregion of the first layer and the region of the third layer therebetweenand the second raised structure comprising the raised feature over thenon-implanted region of the first layer, the second raised structurehaving sidewalls; removing the third mask; forming ohmic contacts onexposed surfaces of the raised features of n-type semiconductor materialand on the exposed p-type implanted region of the first layer; andforming Schottky contacts on the third layer between the raised featuresover the p-type implanted region, on the non-implanted portion of thefirst layer adjacent the second raised structure and on material of thethird layer on the sidewalls of the second raised structure.
 22. Themethod of claim 21, wherein the second layer of n-type semiconductormaterial is more heavily doped that the first layer of n-typesemiconductor material.
 23. The method of claim 21, wherein the secondlayer of n-type semiconductor material has a dopant concentrationgreater than 5×10¹⁸ cm⁻³ and a thickness of 0.5 to 1 μm.
 24. The methodof claim 21, wherein the first layer of n-type semiconductor materialhas a dopant concentration of 2×10¹⁴ to 2×10¹⁶ cm⁻³ and a thickness of 5to 350 μm.
 25. The method of claim 21, wherein the p-type implantedregion in the first layer has a thickness of 0.1 μm or greater and adopant concentration or 5×10¹⁸ cm⁻³ or greater.
 26. The method of claim21, wherein the third layer of n-type semiconductor material has adopant concentration of 5×10¹⁵ to 5×10¹⁷ cm⁻³ and a thickness of 0.2 to1.5 μm.
 27. The method of claim 21, wherein the fourth layer of n-typesemiconductor material has a dopant concentration greater than 5×10¹⁸cm⁻³ and a thickness of 0.2 to 1.5 μm.
 28. The method of claim 21,wherein the n-type substrate has a dopant concentration greater than1×10¹⁸ cm⁻³.